Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis”, by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system, and combined with multiple additional such signals may be used to reconstruct an MR image using a computer and known algorithms.
MR images may be created by applying currents to the gradient and RF coils according to known algorithms called “pulse sequences”. A pulse sequence diagram may be used to show the amplitude, phase and timing of the various current pulses applied to the gradient and RF coils for a given pulse sequence. The selection of a pulse sequence determines the relative appearance of different tissue types in the resultant images, emphasizing or suppressing tissue types as desired. When a pulse sequence is “played out” (i.e., performed, or applied), multiple MR signals are acquired and stored for later reconstruction into an image. Higher resolution images require the collection of more MR signals than lower resolution images.
A pulse sequence uses information about the position and orientation of a target volume in the patient to be scanned that is provided by a scanner operator prior to the start of the pulse sequence. Typically, the scanner operator enters the position and orientation of the target volume using coordinates in the reference frame of the MR scanner (i.e., the coordinate system defined by the direction of the main magnet field and the gradient fields) and the scanner translates these physical coordinates into an “offset vector” and a rotation matrix. The offset vector gives the position of the target volume in logical space and the rotation matrix describes the orientation of the logical space relative to the physical space. Logical space is the coordinate system defined by the slice, frequency, and phase directions of the pulse sequence and may take on any orientation relative to the physical space. The word “offset” is used to describe the center position of the target volume, and refers to the offset of the target volume from the center of the MRI scanner, i.e., the center of the physical space. Information about the size of the target volume is also provided by the scanner operator. The target volume may be scanned using a pulse sequence corresponding to acquisition of a single slice, multiple slices, or a volume, or multiple volumes. Typically, each image is acquired using a single offset vector and rotation matrix. In an acquisition with multiple slices, each slice may each have its own unique offset vector and rotation matrix. For a stack of slices, or a volumetric acquisition, each slice (or image) shares a common rotation matrix and two elements of the offset vector, but the third element of the offset vector is unique to the slice.
For very fast pulse sequences, such as echo-planar imaging (EPI), sufficient data to reconstruct an image may be acquired in much less than one second. However, for most other pulse sequences, acquisition of sufficient data for an entire image requires longer than a minute. For most clinical imaging, multiple slices or a volume are obtained as a single image acquisition, which may take several minutes. Most pulse sequences and reconstruction algorithms are premised on an assumption that the target volume for imaging remains stationary during the image acquisition. Patient motion during the image acquisition may therefore result in image artifacts, and/or may degrade overall image quality. While the assumption of a stationary patient is usually valid for an EPI acquisition, it is not always reliable for longer acquisitions. Some patients are able to remain reasonably still while a pulse sequence is being played out, however, motion may present a significant challenge in pediatric patients, trauma patients, or patients who are unable to follow instructions to remain still. Accordingly, it would be desirable to provide a method and apparatus for acquiring MRI data while a patient is moving during the data acquisition period.